In a cave in New Zealand, scientists are tuning up a giant laser gyroscope to keep track of Earth's rotation. The device, a block of glass 1.2 meters on each side and threaded by laser beams, will measure tiny variations in the rotation rate, which lengthen or shorten the day by a few milliseconds. By keeping track of these vagaries, the gyro could help scientists understand how they are caused by tides, atmospheric changes, and perhaps movements in Earth's core.

Earth scientists can already detect these minute fluctuations by techniques such as very long baseline interferometry, in which two radiotelescopes thousands of kilometers apart monitor slight changes in the apparent position of beacons in the distant universe, called quasars. But the laser gyro, built by scientists from the Federal Office for Cartography and Geodesy in Frankfurt, Germany; the Technical University of Munich; and the University of Canterbury, New Zealand, promises to be much faster, and just as accurate.

The $1 million device, a larger and more sensitive version of those used for aircraft navigation, contains two counterrotating laser beams. The beams interfere with each other to produce a so-called standing wave--a fixed pattern of several million bright and dark spots, called nodes and valleys--crammed into the ring. "This standing wave is standing still in absolute space," says Hans Bilger of Oklahoma State University in Stillwater, a ring laser expert who helped design the gyro, but "if you rotate the physical housing of this standing wave ... you see the valleys and nodes walking by you."

Earth's rotation causes 79 nodes of the standing wave to pass by any point on the gyro every second. To detect changes in Earth's rate of rotation, the researchers must measure variations in this node rate to an accuracy of one in 10 million. "We are getting toward parts per million, and I believe we shall be there in a few weeks' time," says theoretical physicist Geoffrey Stedman of the University of Canterbury in Christchurch, New Zealand. Ultimately, he says, "we hope it will be able to pick up something like [the effect of] lunar tides." That would require a sensitivity of four parts in 100 million, he says, which he calls "quite a tall demand."